Genetics and EvolutionMeiosis - The Genetics of ReproductionAs mentioned in previous pages, the genetic information found in DNA is essential in creating all the characteristics of an organism. This remainsthe case when passing genetic information to offspring, that can occur via a process called meiosis where four haploid cells are created from theirdiploid parent cell.For a species to survive, and genetic information to be preserved and passed on, reproduction must occur. This can be done by passing on theinformation found in the chromosomes via the gametes that are created in meiosis.Chromosome ComplementHumans are diploid creatures, meaning that each of the chromosomes in our body are paired up with another.Haploid cells possess only one set of a chromosome. For example, a diploid human cell possesses 46 chromosomes and a gamete created by ahuman is haploid possesses 23 chromosomes.Tetraploid organisms possess more than 3 sets of a particular chromosome.ReproductionReproduction occurs in humans with the fusion of two haploid cells (gametes) that create a zygote. The nuclei of both these cells fuse, bringingtogether half the genetic information from the parents into one new cell, that is now genetically different from both its parents.This increases genetic diversity, as half of the genetic content from each of the parents brings about unique offspring, which possesses a uniquegenome presenting unique characteristics. Meiosis as a process can increase genetic variation in many ways, explained soon.The Process of MeiosisThe process of meiosis essentially involves two cycles of division, involving a gamete mother cell (diploid cell) dividing and then dividing again toform 4 haploid cells. These can be subdivided into four distinct phases which are a continuous process1st DivisionProphase - Homologous chromosomes in the nucleus begin to pair up with one another and then split into chromatids (one half of achromosome) where crossing over can occur. Crossing offer can increase genetic variation.Metaphase - Chromosomes line up at the equator of the cell, where the sequence of the chromosomes lined up is at random, through chance,increasing genetic variation via independent assortment.Anaphase - The homologous chromosomes move to opposing poles from the equatorTelophase - A new nuclei forms near each pole alongside its new chromosome compliment.At this stage two haploid cells have been created from the original diploid cell of the parent.2nd DivisionProphase II - The nuclear membrane disappears and the second meiotic division is initiated.Metaphase II - Pairs of chromatids line up at the equatorAnaphase II - Each of these chromatid pairs move away from the equator to the poles via spindle fibresTelophase II - Four new haploid gametes are created that will fuse with the gametes of the opposite sex to create a zygote.Overall, this process of meiosis creates gametes to pass genetic information from parents to offspring, continuing the family tree and the species asa whole. Each of these gametes possess unique genetic information due to situations in meiosis where genetic diversity is increased, all of which iselaborated upon on the next page.Independent Assortment and Crossing OverThe previous page investigates the process of meiosis, where 4 haploid gametes are created from the parent cell. Half the genetic information froma parent is present in these haploids, which fuse with gametes of the opposite sex to create a zygote, with a complete chromosome compliment thatPrint - Biology-Online http://www.biology-online.org/kb/print.php?aid=121 of 14 4/11/14 8:51 PMwill create offspring after prolonged growth.The process of meiosis increases genetic diversity in a species. The sex organs which produce the haploid gametes are the site of manyoccurrences where genetic information is exchanged or manipulated.Independent Assortment of ChromosomesAlleles for a particular phenotype determine what characteristic an organism will express, as with the following example whereChromosome 1 contains an allele for blonde hairChromosome 2 contains an allele for brown hairChromosome 3 contains an allele for blue eyesChromosome 4 contains an allele for brown eyesThe top assortment to the left produces 2 blonde hair/blue eyes gametes while the belowproduces 2 brown hair/brown eyes gametesThe top assortment on theright produces 2 blondehair/brown eyes gametes whilethe below produces 2 brownhair/blue eyes gametesThe above indicates that even though the two homologous chromosomes contain thesame genetic information, the assortment of the chromosomes (the order they lie in) can determine what genetic information is present in each ofthe 4 gametes produced. With 23 chromosomes in a human gamete, their are 223 combinations (8388608 combinations)Crossing OverDuring meiosis, when homologous chromosomes are paired together, there are points along the chromosomes that make contact with the otherpair. This point of contact is deemed the chiasmata, and can allow the exchange of genetic information between chromosomes. This furtherincreases genetic variation.There are also many other ways in which genetic variation is increased in a species gene pool, all of which are described in the following pages.The next page investigates the work of Gregor Mendel, an Austrian monk famous for his work involving monohybrid and dihybrid crossing,alongside the continuation into looking at genetic diversity through meiosis and genetics in general.Crossing Over and Genetic DiversityGregor Mendel, an Austrian monk, is most famous in this field for his study of the phenotype of pea plants, including the shape of the peas on thepea plants.Gregor Mendel's WorkMendel's goal was to have a firm scientific basis on the relationship of genetic information passed on from parents to offspring. In light of this hefocused on how plant offspring acquired the phenotype of their seeds. In this example, there are two choices, round and wrinkled seeds.The plants that were used in the experiment had to be true breeding, i.e. those plants with round seeds must have had parents with round seeds,who in turn had parents producing round seeds etc. This is done to increase the accuracy of results.After successfully producing two generations from these true breeding plants, the following was evidentThe first generation of plants produced all had a round seed phenotype.When these first generation plants were crossed, a ratio of 3 round seeds averaged every 1
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